1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile, and a printer.
2. Description of the Related Art
In an image forming apparatus, such as a copying machine, a facsimile, and a printer, a recording material is conveyed to a registration portion to be aligned with an image (referred to as “leading-edge registration”, hereinafter).
Recently, however, a demand for higher accuracy of leading-edge registration has been increasing in the market. The leading-edge registration by the registration portion does not always meet the demand for the higher accuracy of leading edge registration.
Accordingly, a detection member for detecting timing at which a recording material passes may be provided at the upstream position of a transfer portion for the recording material in a recording material conveyance direction. Based on the output of the detection member, registration control (referred to as “leading-edge registration control”, hereinafter) may be performed at the registration portion by changing the conveyance speed of the recording material.
As the detection member used to perform the leading-edge registration control, it is desirable to use a detection member having high resolution.
In this case, a pair of detection portions including a detection member portion and a prism portion, or a pair of detection portions including a light receiving portion and a light emitting portion may be disposed opposite each other in the recording material conveyance path.
Such a pair of detection portions maybe respectively disposed at a secondary pre-transfer upper guide and a secondary pre-transfer lower guide, which are located in the upstream vicinity of a secondary transfer portion.
Further, for accessibility at the time of sheet jam (referred to as “jam”, hereinafter) processing or at the time of maintenance, a secondary pre-transfer lower guide may be mounted on a secondary transfer outer unit, which can be drawn from an apparatus main body.
Japanese Patent Application Laid-Open No. 5-142906 discusses a structure in which a secondary pre-transfer upper guide is provided on a main body side and a secondary pre-transfer lower guide is provided on a drawable unit side, and in which, when the unit is stored, the unit contacts an image forming apparatus so that the position of the unit is set.
However, the structure including a first detection portion disposed at a first guide, a second detection portion disposed at a second guide, and a first guide which is supported by an image forming unit, and a second guide which is disposed at a drawable unit has the following problems.
Since there are many parts in the conventional positioning structure, it is difficult to assure relative positions of the first and the second detecting portions.
The positions of the first and the second detection portions may be assured by adjustment to align the respective positions thereof. However, similar adjustments are also required at the time of replacement of the detection portions or at the time of replacement of the unit. Thus, the operating time for adjustments becomes long and the operating cost therefor becomes high.